The present invention relates to a process for the production of hollow articles from preforms of a thermoplastic, one open end of which preforms possesses a neck moulded at least partially into its definitive shape.
Numerous processes for the production of hollow articles from a thermoplastic are known which comprise two stages, namely the production of a preform from the thermoplastic used, and the final moulding of this preform, for example by blowing, to give the desired final hollow article.
These processes are commonly used for the production of hollow articles by the so-called injection-blowing technique, in which the intermediate preform is produced by injection moulding.
These processes are also applicable to the production of molecularly oriented hollow articles in which it is often necessary to produce preforms which must undergo a well-defined heat treatment in order to induce orientation stresses into their wall during the subsequent final moulding. Such preforms can be produced by various techniques and especially by injection moulding or blow-moulding.
It is desirable to provide preforms having an open neck moulded at least partially into its definitive shape, because during the information of the preforms the thermoplastic is at an ideal temperature to allow precise moulding of details such as threads, orifice rings and the like, which frequently determine the quality of the hollow articles finally obtained. In fact, during the production of oriented hollow articles the preforms are, after heat conditioning, generally no longer at a temperature which allows precise moulding of such details during the final blowing. The same is often true of the production of non-oriented hollow articles by the injection-blowing technique because the thermoplastic material is cooled during the injection moulding of the preform and during the transfer of the latter to the final blow-mould.
Consequently, it is frequently necessary to blow preforms having an orifice ring moulded into its definitive shape, or having other details such as threads, or even to blow preforms of which the neck is completely moulded into its definitive shape. These preforms, where necessary after a heat treatment, are enclosed in a final blow-mould and brought to their definitive shape by blowing by means of an expansion fluid which is introduced into the preforms through a blowing nozzle which enters the interior of the preforms through their open end. This final blow-moulding can furthermore be preceded by longitudinal stretching of the preforms, for example by means of an internal stretching piston which slides in the blowing nozzle, during the production of oriented hollow articles.
In all cases, the wall of the preforms undergoes longitudinal stretching during the final blowing and regardless of the means employed hitherto, it is found that this stretching results in a deformation of the already moulded part of the neck of the preform. Hence, a part of the advantage achieved by moulding this part during the manufacture of the preform, namely greater precision, is lost.
It is, therefore, an object of the present invention to prevent deformation of the already moulded part of the neck of the preform when preform undergoes longitudinal stretching during the final blowing stage.